A communication device such as a mobile telephone and a wireless LAN is required to maintain linearity of a transmission signal over a wide range of output level and operate at low power consumption at the same time. In this communication device, a transmission circuit that operates with high efficiency and low distortion is used. A conventional transmission circuit will be described below.
As a conventional transmission circuit, for example, there is a transmission circuit (hereinafter referred to as “quadrature modulation circuit”) that generates a transmission signal by using a modulation scheme such as quadrature modulation. Since a quadrature modulation circuit is widely known, the explanation will be omitted. As a conventional transmission circuit that outputs a transmission signal that has high linearity more efficiently than a quadrature modulation circuit, for example, there is a transmission circuit disclosed in patent literature 1. FIG. 1 is a block diagram showing an example of a configuration of conventional transmission circuit 10 disclosed in patent literature 1. In FIG. 1, conventional transmission circuit 10 has amplitude phase extraction section 11, amplitude control section 12, phase modulation section 13, amplification section 14, and output terminal 15.
Amplitude phase extraction section 11 extracts amplitude data and phase data from input data. Amplitude data is input to amplitude control section 12. Amplitude control section 12 supplies voltage to match amplitude data to amplification section 14. Also, phase data is input to phase modulation section 13. Phase modulation section 13 performs phase modulation using the input phase data, and outputs a phase modulation signal. Phase modulation section 13 has to be formed by, for example, a frequency synthesizer. A phase modulation signal is input to amplification section 14. Amplification section 14 amplifies a phase modulation signal depending on voltage supplied from amplitude control section 12. A signal amplified at amplification section 14 is output from output terminal 15 as a transmission signal. The output level of the transmission signal changes output voltage of amplitude control section 12 and can be controlled by being supplied to amplification section 14. In this way, the method of separating amplitude data and phase data from input data and modulating by using these pieces of data is referred to as a polar modulation scheme or a polar coordinate modulation scheme. Also, transmission circuit 10 that performs this method is referred to as a polar modulation circuit (hereinafter also referred to as “polar coordinate modulation circuit”).